Effect of Manganese on the Superconductivity of Aluminum

The superconducting critical fields of pure aluminum and of two AlMn alloys, containing 440 and 900 ppm Mn, have been measured as a function of temperature T from their transition temperatures T-c down to T = 0.07 K. The observed T-c depression can be understood in terms of Kaiser's theory for non-magnetic localized states in superconducting alloys, with an effective Coulomb "coupling constant" N-d(0)U-eff equal to 0.43 for the AIMn alloys. Additional evidence that the Mn impurities do not possess a well defined magnetic moment in aluminum in this temperature range is obtained from the critical field results. In particular, the BCS law of corresponding states is obeyed by these alloys. We conclude that these AlMn alloys can be considered BCS superconductors, with a pairing interaction which is weaker than that in pure aluminum.